i think that is an odd error, you must have a huge number of custom fields for that to occur. I don't think a switch to POST will help as the URL being called is too long .trying a different browser may work as different browsers have different lengths of support. If the reason is the number of custom fields we might be able to consolidate some of you can let me know why you have so many and what you're trying to do.

Check the content of your URL. In my case it appears to have been tampered with. The URL address has had an infinite number added to it. But I don't have a solution unless you can highlight the bad portion and delete it.